Half-life and radiation factsheet:
Radioactivity happens because:
1. Some atoms are unstable, and break apart or loose smaller particles to become more stable atoms – this called the ‘activity’.
2. Activity is measured Becquerel’s.
3. When atoms decay they give off radiation.
4. The amount of time for half the radioactive atoms to decay into stable atoms is called the half life.
5. Radiation ionises atoms ((knocks off an electron from them) which is damaging to DNA.
Types of radiation are:
What is it?
What’s its charge?
What’s a use?
Will be stopped by a few cm of air, or skin. Least penetrating.
Two protons and two neutrons bound together
Used in smoke detectors
Will go through air and thin metal, like aluminium, but not thick or very dense metal like lead.
Measuring the thickness of paper in industry
Will go through thick metal sheets, concrete, or rock. The most penetrating.
A very high energy photon (an electromagnetic wave, like light or radio waves)
Radiotherapy, to kill cancer cells
Natural sources of radiation are everywhere, most are natural radioactive atoms in the air (like radon gas), the rocks and soil (like Uranium ore), or dissolved in water. Some radiation comes from stars or other objects in space, and is called cosmic radiation.
Activity is defined as ‘The number of atoms to decay per second): the equation for this is:
Doses of radiation:
Radiation is measured in two ways:
Absorbed dose, which is measured in Greys (or more often milligreys). The equation for Absorbed dose :
Where D is absorbed dose in Greys, E is the amount of energy the radiation carried in Joules, and M is the mass of the thing absorbing the radiation in kg
Equivalent dose takes into account how damaging each kind of radiation is, by multiplying the absorbed dose by a weighting factor (a unitless number picked by observing how much damage each type does). It’s equation is:
Half-life and radioactivity practice questions:
· Source A gives off radiation that goes through aluminium sheet but is stopped by a lead brick
· Source B gives off radiation that is stopped by aluminium sheet, but travels through several centimetres of air
· Source C gives off radiation that is stopped by several centimetre of air.
Which source is giving of which type of radiation?
2: A chunk of radioactive material has a half-life of twenty three seconds.
a) How many whole half-lives does it go through before its activity has dropped to 6.25% of the original value? Hint: 1 whole half-life takes the activity to 50%, 2 half-lives takes the activity to 25%, and so on – you can draw a table of half-lives vs % of activity if it helps.
b) How many seconds before the activity level drops to 25%?
3: A 4 Kg cat gets hit with 50 joules of radiation.
a) What is its absorbed dose?
b) The radiation is alpha particles, with a weighting factor of 20. What is the cat’s equivalent dose?
4: A radioactive source is made of 10,000,000 radioactive atoms. After 2 seconds 1,000,000 have decayed. What’s its activity in Becquerel’s?